Just a question, answer appreciated

Hi list, How do you increase to a counter without refreshing the whole page. For example if you check site www.gmail.com <http://www.gmail.com/> under “Don't throw anything away.” heading there is counter which is increasing (could be javascript magic) but other example is www.neopets.com <http://www.neopets.com/> where time (seconds) is changing without refreshing the whole page ( it is php script ) Any link or hint or code will be welcome ( even comments ) ;) Thanks in advance. salim